In general, an air intake apparatus for an internal combustion engine in which external gas is introduced into air intake pipes is known. Such an air intake apparatus is disclosed in Japanese Patent No. 3964690, for example.
Japanese Patent No. 3964690 discloses a manifold (air intake apparatus) for a multi-cylinder (four-cylinder) engine in which blow-by gas (an unburned air-fuel mixture) is introduced into air intake pipes. In this air intake apparatus disclosed in Japanese Patent No. 3964690, a cutout through which the blow-by gas is introduced into an air intake pipe of each cylinder is provided. A distribution passage groove that distributes the blow-by gas to each cylinder and the air intake pipe of each cylinder are connected to each other via the cutout of the air intake pipe. The length of the cutout in a direction in which the blow-by gas flows (the length between the distribution passage groove and the air intake pipe) is smaller than the width of the cutout (the width in a direction perpendicular to the direction in which the blow-by gas flows). That is, the blow-by gas is introduced into the air intake pipe from the distribution passage groove via the cutout of a relatively short path.